Little information is available on factors directly controlling the osteoclast or the molecular events responsible for osteoclast formation and bone resorption. The cell and molecular biological characterization of the osteoclast has been severely hampered by the inability to isolate large numbers of osteoclasts or osteoclast precursors and the unavailability of established osteoclastic cell lines. The proposed studies are designed to allow the establishment of cell lines that express the full differentiated phenotype of osteoclasts from osteoclast tumors formed in transgenic mice. These cell lines will then be used to address questions concerning the effects of estrogen and other osteotropic factors on osteoclast function. The proposed strategy involves directing the expression of an oncogene, such as SV40 Tantigen, specifically or preferentially to osteoclast cells using the regulatory regions of the genes encoding tartrate resistant acid phosphatase (TRAP) and the calcitonin receptor (CTR). Transgenic mice expressing these hybrid transgenes are expected to develop bone tumors of osteoclast origin, or possibly immortalized osteoclast precursors in the blood, spleen or bone marrow. These immortalized cells will be placed in culture and cell lines will be established. The cell lines will be extensively characterized to determine the extent to which they resemble osteoclasts or osteoclast precursors and to test their ability to differentiate in culture. In addition, cell lines will be derived from both male and female mice to allow a comparison of phenotype. Finally, since estrogen may affect the formation and activity of osteoclasts, studies will be undertaken to determine the effects of estrogen on the growth and differentiation of these cells and on their ability to resorb bone. In addition, estrogen will be administered to the transgenic mice to determine if the pattern of mononuclear or multinucleated osteoclastic cells is altered in vivo. Based on the success of the transgenic mouse approach for deriving established differentiated cell lines representing other rare cell types, we are hopeful that the proposed studies will lead to the development of established cell lines representing osteoclasts or osteoclast precursors. Such cell lines would provide a novel system for in vitro analysis of important questions concerning the role of estrogen and other osteotropic factors or accessory cells in regulating osteoclast function.